Feeding wild birds is a popular pastime for nature-starved urban dwellers in many countries. But this practice may have unintended consequences, altering the composition of bird communities and favoring introduced species to the detriment of native ones, according to research published yesterday in the Proceedings of the National Academy of Sciences.

In the study, researchers recruited two dozen bird lovers living in a residential area of Auckland, New Zealand. Half of these individuals established feeding stations on their property, and put out a few slices of bread and a cup of birdseed every morning. The other half agreed not to feed the birds in their yard at all for the duration of the 18-month experiment.

Previous studies have identified correlations between bird feeders and the species found in an area, but this is among the first to use experimental manipulations to suss out the effects of bird feeding more directly.

One of the researchers, Josie Galbraith, visited the study sites once a month and counted all of the birds she saw or heard in each area. Overall, Galbraith conducted nearly 600 of these 10-minute bird surveys, recording a total of 18,228 birds belonging to 33 species. Auckland’s urban bird fauna is known to have a preponderance of introduced species, and the surveys turned up 17 introduced species and 16 native ones.

Feeding the birds alters a backyard bird community in favor of the introduced species, the researchers found. Around feeding stations, bird communities are less variable – rather one-note, you might say – compared to non-feeding properties, and tend to be dominated by introduced species.

Lots of house sparrows, spotted doves, and European starlings flocked to the properties that provided food. Meanwhile, these backyards became less suitable habitat for the grey warbler, a New Zealand native. The abundance of that species fell by half on feeding properties during the experiment, while remaining stable in non-feeding backyards.

The researchers speculate that the hubbub of birds around a feeding station may disrupt the grey warbler’s foraging strategy, which involves methodically moving through tree canopies searching out insects.

Many introduced bird species that are common in urban areas around the world are omnivores and grain-eaters, so they’re well placed to capitalize on the kinds of food that people tend to provide. By contrast, most of the native birds common in urban areas in New Zealand eat insects (like the grey warbler does), nectar, or fruit. The effect of bird feeders might differ in other cities where the native birds represent a different mix of guilds, the researchers note.

In large part, the bird community changes seen in the experiment were temporary, reversing after the volunteers stopped providing food. But the authors say bird feeders are so widespread that they probably do shape urban bird communities at a larger scale. In a previous study, for example, the researchers found that about two in five households in urban New Zealand put out food for wild birds. This may be helping the spotted dove, which was introduced to Auckland in the 1920s, become established and spread into the surrounding countryside.

It’s hard to deny the pleasure of having a backyard bird feeder. But the new study suggests that we ought to be a little more honest with ourselves about who really benefits from this practice. The idea that we’re doing it for the birds might be, well, for the birds. – Sarah DeWeerdt | 5 May 2015